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Board of County Commissioners February 19, 1979 Page -2- and any constriction increases the elevation of the flood waters in an'amount. proportionate to the reduction of the cross sectional area. 0".- '44, ..». .,. Normal Stream Flood Same Flood With Fill Many local governments restrict the amount of fill allowed in a floodplain to that which is proven by engineering studies to have no appreciable increase in the 100 year flood elevation. Others simply establish a flood zone in their Zoning Ordinance which prohibits filling or general development within the deter- mined floodplain area. In the case of the Sebastian River, constriction of tidal flow could create higher water levels at high tide downstream from the fill area. This would probably be of a minimal nature because of the small difference in tides in this area. Water quality can be severely degraded by.filling of floodplains if run-off and erosion are not carefully controlled. Fill dirt, being relatively uncompacted, tends to erode easily and is particularly vulnerable to heavy rains or floods before vegetation has properly taken root. This results in sedimentation problems downstream where the velocity of flow slows and soil particles drop out of the stream flow. Enough fill and erosion can eventually fill a stream, river or lake to the point that swimming, fishing and other useful functions become unsuitable. Many governments control this degradation of water quality by ordinance requiring permitting of any dredge or fill operation and restricting the amount of sediment which may flow to the river, Regulations may .require settlement ponds, sediment barriers, bulkheads or other control devices during the construction of fill stage and after completion of the project. Usually, the amount of sediment in run-off going to the stream is limited. by a standard measurement of turbidity. The Sebastian River is the only stream in the eastern portion of Indian River County which would be affected in flood levels by filling. There are areas in the west though which could be significantly affected by filling. In terms of water quality, however, filling or grading anywhere near the Indian or Sebastian Rivers or tributory canals - can significantly influence water quality. Although existing county ordinances do not appear to control fill, staff does recommend adoption of ordinances to establish sedimentation and floodplain stan- dards designed to eliminate flood and water quality problems from filling and grading of land. SR: jt PLANNER REDICK EXPLAINED THAT TWO MAJOR THINGS HAPPEN WHEN FILLING IN A FLOOD PLAIN AREA: IF YOU RESTRICT THE FLOW OF WATER THROUGH THE FLOOD PLAIN AREA, YOU INCREASE THE FLOOD LEVEL; AND IF YOU BUILD IT UP AND CONSTRICT IT DOWN TO THE CHANNEL ITSELF, YOU ARE GOING TO RAISE THE ELEVATION OF FLOOD AREA. HE CONTINUED THAT UNLESS THE FILL SITUATION IS CAREFULLY CONTROLLED, A GREAT DEAL OF SEDIMENT WILL GET INTO THE STREAM, 37 FEB 2 11979 Bou 39 PAGE 1.1.8